1. Field of the Invention
The present invention relates to medical instruments generally and more particularly to a preamplifier of reduced size for use therewith.
2. Description of the Prior Art
Evoked-potential-type medical instruments are of increasing importance as diagnostic tools. Such instruments test a portion of a subject's neurological system. More particularly, such instruments stimulate, at a periodic rate, one of the subject's senses (hearing, sight, touch, etc.). Further, such instruments receive physiological (electroencephalograph (EGG), electromyography, and/or electrocardiography (EKG)) potentials generated by the subject. Finally, such instruments synchronously detect the received potentials to separate therefrom those potentials which are generated by the subject in response to the stimulus, in other words, the evoked potentials. Based upon the characteristics of the evoked potentials, including their amplitude, shape, and delay, deductions may be made as to the integrity of the specific neurological system and as to the presence of certain diseases which affect it.
For coupling physiological potentials generated by the subject to the instrument while electrically isolating the subject from the instrument, medical instruments commonly employ a number of (instrumentation-type) preamplifiers. Typical thereof is the prior-art-type preamplifier which is illustrated in FIG. 1 and which is designated generally by the number 10. Preamplifier 10 is shown to include as principal components three operational-type amplifiers, which are respectively designated 20, 22, and 24 and an optical isolator, which is designated 26. Operational amplifier 20 is configured with the non-inverting input of the amplifier connected to a line 28, the inverting input of the amplifier connected to a node 30, and the output of the amplifier connected to a node 32. Operational amplifier 22 is configured with the inverting input of the amplifier connected to a node 34, the non-inverting input of the amplifier connected to a line 36, and the output of the amplifier connected to a node 38. Node 30 is both coupled to node 34 by a resistor 40 and coupled to node 32 by another resistor 42. Node 34 is also coupled to node 38 by still another resistor 44.
Operational amplifier 24 is configured with the amplifier inverting input both coupled to node 32 by a resistor 50 and coupled to a node 52 by a resistor 54, with the amplifier non-inverting input both coupled to node 38 by a resistor 56 and coupled to a node 58 by a resistor 60, and with the amplifier output connected to node 52. Node 52 is coupled to node 58 by the series combination of a resistor 62 and the input portion of optical isolator 26, the output portion of which is connected between a pair of lines 64 and 66. Node 58 is connected to circuit ground.
Resistor 40 has, typically, a resistance of 10K ohms. Typically, resistors 42 and 44 each have a resistance of 1 meg ohm; resistors 50 and 56 each have a resistance of 10K ohms, and, resistors 54 and 60 each have a resistance of 100K ohms.
When lines 28 and 36 are connected each to a respective one of a pair of electrodes that are suitably applied to the subject, an electrocephalograph potential, which is generated (differentially) between the electrodes by the subject, is received by preamplifier 10. Preamplifier 10, first, amplifies the (voltage) level of the received potential. With the abovementioned resistor resistances, operational amplifiers 20 and 22 develop between nodes 32 and 38 an amplified potential the (voltage) level of which is approximately 200 times that of the received potential developed between lines 28 and 36. (More specifically, at least to the extent that the amplifier approaches the ideal, operational amplifier 20 maintains the potential at node 32 at whatever potential is required to maintain the potential at node 30 at the same level as the potential which is developed on line 28. Also, operational amplifier 22 maintains node 34 at the line 36 potential. Thus, a potential is developed across resistor 40 which is similar to the potential that is developed between lines 28 and 36. Since, ideally, the currents flowing into or out of the non-inverting inputs of amplifiers 20 and 22 are negligible, the current which flows through resistor 40 must flow through resistors 42 and 44. This current develops a potential across resistor 42 which is equal to the resistor 40 potential times the ratio of the resistance of resistor 42 to that of resistor 40. A similar potential is developed across resistor 44.)
Operational amplifier 24 of preamplifier 10 further amplifies the (voltage) level of the received potential. With the above-mentioned resistor resistances, operational amplifier 24 develops between node 52 and node 58 (circuit ground) a further amplified potential the (voltage) level of which is approximately 10 times that of the amplified potential developed between nodes 32 and 38.
Finally, resistor 62 developes from the further amplified potential a current of suitable level for driving the input portion of optical isolator 26. From the current, optical isolator 26 develops between lines 64 and 66 a potential suitable for driving other portions of the instrument while suitably isolating therefrom the subject.
The above-mentioned prior-art-type preamplifier is disadvantageous for a number of reasons. For one, the relatively large voltage gain provided by operational amplifiers 20 and 22, and, even, operational amplifier 24, is at the expense of band width. As a consequence, preamplifier 10 has relatively poor (narrow) band width characteristics.
Also, preamplifier 10 has relatively poor common mode rejection characteristics. The relatively poor common mode rejection characteristics are due, at least in part, to the use of a reference potential, circuit ground potential (or any potential not related to the lines 28 and 36 potential). Also, common mode rejection characteristics suffer unless precision, matched and/or adjustable resistors are employed.
Additionally, preamplifier 10 is relatively noisy.
Further, preamplifier 10 is relatively complex and, as a consequence, relatively large. When combined with many other such preamplifiers that are required of some instruments, a package results which may be difficult to locate relatively near the subject. This is particularly the case where space is at a premium, such as in an operating room. Yet, the particularly electrically noisy environment of an operating room makes locating the preamplifier relatively near the subject even more important.
Finally, preamplifier 10 lacks the means for protecting the preamplifier from destructive potentials of the type which are generated when electrical-discharge-type (Bovie) knives or defibrillators are used.